We are studying the mechanism by which a single genetic locus, the mating type locus (MAT) determines the cell type of yeast (either a or Alpha). Based on genetic and physiological arguments, we proposed that MAT codes for regulatory proteins that turn on or turn off other genes located elsewhere in the yeast genome. As a direct test of this hypothesis, we have cloned some of the genes putatively under MAT control. We find for at least one of the cloned genes (STE3) that it is indeed controlled just as predicted by our earlier hypothesis: production of the STE3 RNA requires products of the Alpha mating type locus for its expression. Our current studies are directed towards determining the precise mechanism by which MAT regulates this process. One of the functions specific to Alpha cells (which is not produced by a cells or by a/Alpha diploids) is the oligopeptide mating factor, Alpha-factor. We hypothesized that the gene coding for Alpha-factor is controlled by the mating type locus. In addition, because Alpha-factor is only 12 amino acids in length, it has been proposed that it is produced in a precursor form. To provide information on these two points, we have cloned the Alpha-factor structural gene using an indirect strategy. Proof that the cloned DNA segment contains the Alpha-factor structural gene comes from the partial nucleotide sequence, which shows that indeed it codes for the known Alpha-factor sequence. Further nucleotide sequencing will reveal whether there is a precursor and give information on its size. Other areas of investigation include studying the mechanism by which genetic informating at the mating type locus (a genetic "cassette") is removed and replaced by a cassette of another type. One line of study involves cloning of the HO gene, which catalyzes this cassette substitution process. HO has been cloned and is now under further investigation.